If you want to form a question, how/what/which etc. should be followed by a verb. And here, the verb is 'can'.
So,
How can we achieve this?
forms a question.
Take another example.
How we do it
is not forming a question. You need an auxiliary verb there to form a question.
How do we do it?
I often teach my daughter in this way...
The first one is 'actually' a sentence and not a question.
[This is] how we can achieve this!
It's not possible to mark it as a sentence with an auxiliary verb in it placed after 'how'
[This is] how can we achieve this?
The latter requires question mark and the former does not!
Just a trick!